It is known to detect toxic gases using chemical cassettes having a paper tape substrate impregnated with a specific chemical material; the paper is usually absorbent and is impregnated with the chemical material by dipping it into a trough of a solution of the material. The tape is subsequently dried and installed in a cassette. During use, the paper tape is placed in an analyser that draws gas from the atmosphere being monitored through the tape. The chemical material absorbed on or in the tape is chosen to react with the target gas and to change colour; the degree of colour change, for exposure to a predetermined amount of atmospheric gas, provides a measure of the concentration of the target gas in the atmosphere being monitored. The analyser detects the change of colour, i.e. radiation absorbed or transmitted by the reaction product at a wavelength at which the unreacted colour change material absorbs or transmits to no extent or a lesser extent, and then calculates the gas concentration by comparing the colour change to a table of known gas responses that has been pre-programmed into the analyser. Periodically, the tape is advanced to bring a fresh length of tape to the position within the analyser through which the target gas is drawn to make a further measurement using the fresh length of tape. One major advantage of the tape system is that a permanent visual record exists of positive gas responses as seen by the visible colour change of the tape.
The colour-change response will give a measure of the amount of the target gas in the atmosphere being monitored. However the colour-change response will depend on the amount of colour-change material absorbed in or applied onto the tape, which in turn will depend on the amount of colour-change material applied to the substrate during manufacture and this can vary, depending on the control applied by the manufacturer. Secondly, the tape and the colour-change to material can degrade over time under conditions of temperature, humidity and/or light. Therefore the amount of colour-change material present will depend on the time that has elapsed since manufacture. Finally, the substrate is not necessarily uniform, either in its density or its absorptivity and different areas can absorb different amounts of the colour-change material. All these factors can lead to inaccuracies in the response to a given concentration of target gas both from tape to tape but also along the length of a single tape.
It is an object of the present invention to overcome or alleviate some or all of the above problems.